Enzymes for degrading aromatic compounds have potential commercial application in the pulp and paper industry, the production of fuels and chemicals from lignocellulose, the enhancement of livestock feeds, and the bioremediation of aromatic hazardous wastes.
Lignin is a complex polymer of phenyl propanoid units with a variety of interunit linkages forming a nonlinear, random structure. Lignin comprises 10-35% of the dry weight of lignocellulose-rich materials such as wood, straw, and corn stover. Lignin is resistant to biological destruction, although it is enzymatically degraded by various higher order fungi. In nature, the basidiomycetes that cause white-rot wood decay are major degraders of lignocellulose. White-rot fungi oxidize lignin completely to carbon dioxide. Extracellular enzyme complexes secreted by these fungi catalize oxidative reactions of the lignin structure. White-rot fungi have also been shown to oxidize and degrade a wide range of other aromatic structures including a variety of man-made, toxic aromatic compounds. The term xe2x80x9cwhite-rot fungixe2x80x9d as used herein is intended to include fungi having enzymes capable of oxidizing and thereby degrading aromatic compounds.
There are an estimated 1700 species of white-rot fungi. However, research on enzymatic lignin degradation has concentrated on one organism: Phanerochaete chrysosporium. Lignin-degrading enzymes from this organism have been purified and characterized. A large volume of research literature describes processes for growing P. chrysosporium in liquid media for lignin degradation or production of lignin-degrading enzymes. The conventional production of lignin-degrading enzymes in liquid media occurs during secondary metabolism and is initiated by nitrogen or glucose starvation. For instance, in U.S. Pat. No. 4,554,075, Chang et al. describe a process for growing white-rot fungi by carrying growth into secondary metabolism wherein nitrogen starvation occurs. See also Ming Tien in an article in CRC Critical Reviews in Microbiology, titled xe2x80x9cProperties of Ligninase From Phanerochaete Chrysosporium and Their Possible Applicationsxe2x80x9d, Volume 15, Issue 2 (1987) at p. 143 and U.S. Pat. No. 4,891,230 to Aust et al.
The slow growth rates and low cell mass production associated with starved cultures results in long growth times and low yields thus making this impractical for commercially producing enzymes for pretreating wood pulp in paper making processes, for in situ treatment of toxic waste, or for enhancing lignocellulose for livestock feed. Tien notes on page 144 in the same article listed above that scale-up from liquid culture grown in flasks has proven difficult.
To overcome the low cell mass production, the art has suggested growing several species of white-rot fungi using solid culture media in solid state reactors. In these instances, the fungus grows on a substrate of moist solid lignocellulose-containing materials. Straw, several types of wood, and milled corn cob have been disclosed as substrates in the literature. These materials have been selected as culture substrates primarily because they are typical of the materials degraded by the white-rot fungi in nature. They have a relatively high lignin content of 10-35%, low nitrogen levels, and limited access to cellulose as a carbon source. White-rot fungi can be grown in such solid-state cultures, but obtaining lignin-degrading enzymes in cell and solids free extracts of such cultures has proved an elusive task as the enzyme activity remains bound to the substrate.
Several patents as well as other literature disclose processes for preparing ligninase in solid cultures including U.S. Pat. No. 4,711,787 to Odakra, which describes using okra as a substrate for the production of livestock feed. Rolz, et al., in an article in Applied Microbiology and Biotechnology titled, xe2x80x9cWhite-Rot Fungal Growth on Sugarcane Lignocellulosic Residuexe2x80x9d, Volume 25 (1987) pp. 535-541, report using sugarcane residue as a substrate. In U.S. Pat. No. 4,891,320, Aust et al. list as typical materials used to grow white-rot fungi for use in degradation of aromatic compounds shredded paper, wood shavings, sawdust, corn cobs, and humus. None of these references discloses the production of enzymes during the primary metabolic growth phase or the production of cell-free extracts of the culture containing lignin-degrading enzymes.
It is believed that the reason why extracting cell-free enzymes is difficult in conventional solid state processes for producing enzymes is that the enzymes are absorbed into the lignocellulosic substrate materials. Thus, when using substrates of the type normally associated in nature with white-rot fungi, lignin-degrading enzymes are difficult to extract or purify in active form. These substrates typically have a high lignin content and low protein content. On the other hand, small amounts of cell-free enzymes are present in liquid cultures, presumably because there are no surfaces for enzyme absorption.
Both liquid and solid substrate cultures of white-rot fungi have been the subject of at least 15 years of intensive research in numerous laboratories, as evidenced by the volume of research literature and patents granted in this field. However, the problems of producing enzymes during the primary metabolic growth phase, of producing cell-free enzymes from solid culture and of producing lignin-degrading enzyme preparations with commercially useful enzyme concentrations remain unsolved.
This invention pertains to a process for growing white-rot fungus in a solid state culture using sugar beet pulp as a substrate. The fungus can be grown for the production of by-products of fungal growth such as enzymes for degrading aromatic compounds such as lignin. The process advantageously permits the production of lignin-degrading enzymes during the primary metabolic growth phase of the fungus rather than during secondary metabolism, and the easy separation of lignin-degrading enzymes from the substrate material for the production of cell-free enzymes preparations.
According to the process, white-rot fungi are grown under growth supportive conditions on a sugar beet pulp substrate. A substrate of sugar beet pulp for growth of white-rot fungi may be prepared by, e.g., sterilizing the raw pulp as by autoclaving and then cooling and adding water or nutrient solutions. The substrate then is inoculated with what may be a conventional white-rot fungi inoculum. The inoculated substrate is then placed in a solid state reactor for growing fungi and the mixture is aerated to enhance growth.
The method of the invention is particularly advantageous for the production of lignin-degrading enzymes. The growth of white-rot fungi on sugar beet pulp substrate results in production of lignin-degrading enzymes during the primary metabolic growth phase of the fungi when an abundance of nutrients are available. This is in contrast to conventional culture systems where production takes place during secondary metabolism when nitrogen or carbon may be restricted.
At the conclusion of the growing period, the substrate can be washed with water to bring enzymes into solution. The lignin-degrading enzymes are not irreversibly bound to the substrate using this process. The enzyme-rich solution can be centrifuged and filtered to provide an active, cell-free liquid enzyme preparation containing lignin-degrading enzymes that have been removed from the substrate.
The ability to commercially produce lignin-degrading enzymes during the primary metabolic growth phase and further to produce cell-free lignin-degrading enzymes is an advantage of this invention over conventional solid state or liquid culture process used to produce these enzymes using white-rot fungi.
Sugar beet pulp is used as the substrate material for fungal growth in accordance with this invention. Sugar beet pulp is produced in large amounts and is readily available for high-volume, commercial applications for growing white-rot fungi.
Sugar beet pulp has not been reported as a natural substrate for white-rot fungi. It has a relatively low lignin content of 1% to 3%. White-rot fungi occurs naturally as decay organisms on woody materials with high lignin content such as okra, sugarcane, shredded paper, wood shavings, sawdust, corn cobs and humus. These materials have been used in conventional production of lignin-degrading enzymes.
Sugar beet pulp contains 8-10% protein and up to 5% residual sucrose and is not a carbon and nitrogen limited substrate. Yet, white-rot fungi produce lignin-degrading enzymes when grown on sugar beet pulp during the primary metabolic growth phase. Lignin-degrading enzymes are produced by white-rot fungi when grown on sugar beet pulp supplemented with glucose and the additional nitrogen sources peptone (a soluble protein hydrolysate) and yeast extract. This result is unexpected because production of these enzymes using conventional processes typically occurs only with nitrogen or carbon starvation during secondary metabolism.
Sugar beet pulp is a byproduct of the processing of sugar beets for sugar (sucrose). In a typical process, sugar beets are sliced and extracted with hot water to recover the sugar. Sugar beet pulp is the residue of sugar beets remaining after the extraction process. In most sugar beet processing plants, the sugar beet pulp is dried and sold as cattle feed. Sugar beet pulp is composed of the following constituents with the typical proportions shown as a percentage on a dry weight basis.
Particles of sugar beet pulp are typically 0.5 to 1 cm in the largest dimension and irregularly shaped.
Sugar beet pulp can be prepared for use as a solid culture substrate as follows. Dry sugar beet pulp is moistened with one of a number of standard nutrient solutions supportive of fungal growth and then sterilized by autoclaving, e.g., at 125xc2x0 C., 15 psi for 20 minutes. Other generally accepted methods for sterilization can be used involving different temperatures, pressures, and durations as long as the sugar beet pulp is sterilized before inoculation. The sugar beet pulp is then cooled to between 20-40xc2x0 C.
An inoculum of white-rot fungi is then aseptically and thoroughly mixed with the cooled sugar beet substrate. The inoculum can be prepared in any conventional manner such as by first selecting a pure culture of a white-rot fungus and maintaining this fungus on nutrient agar slants. Next, the culture on the agar slants is transferred to either a liquid or solid media and grown at 20-40xc2x0 C. The media selected varies somewhat depending upon which organism is selected for growth. If a liquid media is selected for growing the inoculum, the liquid inoculum media should contain glucose, a nitrogen source, and nutrient salts. Liquid cultures can be held stationary or agitated during the culture growth phase. If a solid media is selected for growing the inoculum, either sterilized sugar beet pulp, prepared as described above, or other known materials can be used as a substrate. Generally, sufficient inoculum culture is grown to provide approximately 1-20% by volume of the mass of substrate to be inoculated.
According to the present invention, the inoculated sugar beet pulp comprises a solid state culture characterized by a solid phase of particles of sugar beet pulp, an aqueous phase sorbed into the particles of the pulp and a gas phase in the interparticle spaces. Moisture content of the sugar beet pulp is 40 to 80%, typically 66% by weight. Optionally, 2-10% sterilized straw can also be added to the sugar beet pulp. Straw may be added before or, more typically, after the beet pulp is wetted. The straw improves the physical characteristics of the solid culture by increasing the volume and maintaining integrity of interparticle spaces resulting in improved aeration, temperature control, and moisture control.
The fungus grows on the surface of, and penetrates into, the particles of sugar beet pulp.
The inoculated substrate is placed in a vessel designed as a solid culture reactor or in a trench or pile. The shape and dimensions of the vessel used as the solid culture reactor may be varied widely. In one currently developed embodiment, the inoculated substrate is placed in cylindrical or rectangular vessel in a bed approximately 70 cm deep. The vessel is designed so that air at controlled temperature and humidity can be circulated through the bed and appropriate means are provided for this.
In a solid state reactor, the temperature, nutrients, aeration rate, and growing period can be varied to regulate the metabolic rate of the fungus. Metabolic conditions also can determine the specific types of lignin-degrading enzymes produced by the fungus. Typically, the temperature of the substrate is maintained between 20-40xc2x0 C. depending on the organism and enzyme preparation being produced. A nutrient solution may be added to the substrate as necessary to maintain primary metabolic growth phase. Sufficient conventional nutrient solution is provided during the growing period to prevent nitrogen or carbon starvation or secondary metabolism.
An atmosphere of air, or an artificially created atmosphere having an oxygen concentration of 7-100%, is circulated through the substrate during the growing period. An aeration rate of between 0.05 to 20 unit volumes of air per minute per unit volume of substrate may be used. The aeration atmosphere preferably is maintained between 70-99% relative humidity. The relative humidity typically is varied to maintain the absorbed water content of the substrate between about 40-80% initially, and then between about 60-80% at the end of the growing period, with 66-72% being typical. The growing period of the culture is varied from 4 to 30 days, depending on the identity of the organism and the type of enzyme to be produced.
At the completion of the growing period, the culture comprises a fungal cell mass, unutilized culture substrate, and extracellular enzymes. For some applications, particularly is situ degradation of toxic wastes, the whole wet culture may be used without further processing by merely turning the culture into the soil.
To produce a cell-free liquid enzyme preparation containing lignin-degrading enzymes, one can extract the culture by mixing it with water. Alternatively, water together with conventional, biologically compatible detergents, such as TWEEN 80, may be used as an extractant. A cell-free solution containing lignin-degrading enzymes can be produced by mixing the culture with the extractant, then centrifuging and filtering to remove all cells and solids (with, for example a 0.8 micron filter).
The sugar beet pulp substrate is capable of sustaining growth of a variety of white-rot fungi to induce production of at least four types of enzymes, namely, peroxidases, manganese peroxidases, oxidases and laccases. To determine the nature of the enzymes present in various extracts, conventional assay procedures such as those based on enzymatic oxidation of compounds such as phenol red, veratryl alcohol, vanillylacetone and anis alcohol with and without the presence of hydrogen peroxide or oxygen or manganese are used.
Assays of peroxidase are based on oxidation of phenol red or veratryl alcohol in the presence of hydrogen peroxide. See e.g., Tien, M. (1987) Critical Review in Microbiology 15(2):144; Farrell, R., U.S. Pat. No. 4,687,741; Kuwahare, M. et al. (1984) FEBS Letters 169(2):247-250; Walder, R. et al. (1988) Applied Microbiology and Biotechnology 29:400-407. Assays for manganese peroxidase measure oxidation of phenol red, veratryl alcohol or vanillylacetone with the presence of both hydrogen peroxide and manganese. See Kuwahare, M. et al. and Walder, R. et al., supra; Bonnarme, P. and Jefferies, T. W. (1990) Applied and Environmental Microbiology 56(1):210-217. Assays for oxidase are based on oxidation of veratryl alcohol or anis alcohol with the presence of oxygen. See Muheim, A. et al. Enzyme and Microbial Technology; Walder, R. et al., supra. Assays of laccase activity is based on oxidation of phenol red or 2,6-dimethoxy phenol in the absence of hydrogen peroxide and manganese. See Kuwahare, M. et al. and Walder, R. et al., supra; Haars, A. and Huttermann, A. (1980) Archives of Microbiology 125:233-237.
As illustrated in the examples below, culture extracts grown by the processes of this invention have been assayed using each of these procedures. The presence or absence of hydrogen peroxide, manganese, and oxygen in the enzyme reaction provides a basis for distinguishing the different types of activities.
It is an important feature of the invention that all of these different types of enzymes can be produced. Different commercial applications may require specific types or combinations of these types of enzyme activities. Furthermore, the different types of enzymes produced by various white-rot fungi grown by this process, differ in substrate specificity, pH optima, buffer requirements and stability. These differences may confer relative advantages of one organism and or one type of enzyme in specific commercial applications.